The hereditary breast and ovarian tumor suppressor protein BRCA1 plays critical roles in DNA repair, cell cycle checkpoint control, and maintenance of genomic stability. We have recently identified several new regulators of BRCA1, which form a novel complex that is involved in the DNA damage induced ubiquitin signaling for BRCA1 recruitment to the DNA damage sites. We named this complex the BRCA1 A complex. The BRCA1 A complex contains at least five different components, Rap80, Abra1, NBA1, BRE and BRCC36. It associates with BRCA1 through an interaction of Abra1 with the BRCA1 C-terminal BRCT domains. We found that four members of the BRCA1 A complex contain ubiquitin-binding motifs and that BRCC36 is a deubiquitinating enzyme. Interestingly the BRCA1 A complex shares striking similarities to the lid of the 19S proteasome regulatory complex. However, the exact role of this complex at DNA damage sites is not known, nor the role of the complex in BRCA1 signaling and tumor suppression is known. We hypothesize that the BRCA1 A complex functions as a deubiquitinating complex to further modify the DNA damage induced poly-ubiquitination at sites of damage for DNA repair and BRCA1 signaling. To test this hypothesis and to determine the potential tumor-suppressing function of the BRCA1 A complex, we will first examine the assembly and function of this complex as a deubiquitinating complex. Secondly we will determine the role of this complex in DNA damage repair and signaling. Finally we will determine whether the BRCA1 A complex plays a critical role in maintaining genomic stability and tumor suppression in vivo through investigations of Abra1-deficient mice. Together, our studies will provide new avenues for understanding the role of BRCA1 in DNA damage response and tumor suppression.